As is known in the art, high electron mobility transistors (HEMTs) generally include a substrate having a channel layer located between a buffer layer adjacent to the substrate side of the HEMT and a barrier layer. The barrier is composed of a different material than the channel layer and a heterojunction is formed at the channel layer/barrier layer interface with a highly conductive two-dimensional electron gas (2DEG) residing on the channel side of the heterojunction or interface. Generally the bather layer has a wider bandgap than the channel material and charge is transferred to the channel layer from the addition of donor impurities and/or polarization differences at the heterojunction. The buffer layer can either be of the same material as the channel layer or a different material. Structures where the buffer layer and channel layer are of the same material are referred to as single heterojunction structures. Structures where the buffer layer and channel layer are of different materials are referred to as double heterojunction structures or double channel HEMT structures.
Indium Nitride (InN), Gallium Nitride (GaN), Aluminum Nitride (AlN), Boron Nitride (BN) and all of their associated alloys including Inx(AlyGa1-y)1-xN (where 0≤x≤1 and 0≤y≤1) and Bz(Inx(AlyGa1-y)1-x)1-zN (where 0≤x≤1 and 0≤y≤1 and 0≤z≤1) are a family of materials known as Group III-Nitrides. The Group III-Nitrides are used in power and microwave transistor device electronics in part because of their breakdown voltages, bandgap energies, and saturation velocities. One of the principal issues related to device performance is the crystal quality of the epitaxial material grown from Group III-Nitride materials as most epitaxial Group III-Nitride heterostructures are grown on lattice mismatched substrates due to the high cost and low availability of large diameter bulk Group III-Nitride substrates. The lattice mismatch results in the formation of numerous defects detrimental to device performance. Typically Group III-Nitride buffer layers are grown in excess of one micron in thickness before the active device regions are grown to allow the material to relax and to reduce as many defects as possible through the growth process.
Group III-Nitride materials in the wurtzite crystal structure exhibit spontaneous and piezoelectric polarization due in part to structural deviations from the ideal tetrahedral coordination along the (0001) axis (c-axis) and differences in the electronegativity between the bonded group III and nitrogen atoms. When the crystal undergoes a distortion due to an applied strain that changes the axial ratio (ratio of the lattice constants c/a), the polarization of the crystal is impacted.
Many Group III-Nitride device structures take advantage of the polarization mismatches that occur when two different Group III-Nitride materials are epitaxially bonded together to produce electrically active carriers at these heterojunctions. The polarization mismatch at the heterojunction induces a sheet charge at the heterojunction. If the polarization induced sheet charge density is positive, free electrons will tend to compensate the polarization induced charge, and if the band offsets at the junction are large enough, a two-dimensional electron gas or 2DEG will form as in the case of the AlGaN/GaN heterojunction.
If the two Group III-Nitride materials comprising a heterojunction are fully relaxed with their respective bulk equilibrium lattice constants, then the difference in the total polarization mismatch is equal to the differences in their respective spontaneous polarizations. If the in-plane lattice constant mismatch between an epitaxial layer and an underlying layer at a heterojunction results in the expansion or contraction of one of the layers, the induced piezoelectric polarization will also contribute to the total polarization.
As is also known in the art, there is a need for heterojunction Group III-Nitride HEMTs which confine the 2DEG charge in the channel layer, while maintaining low off-state leakage and good Radio Frequency (RF) performance. Typically in single heterojunction Group III-Nitride based HEMT structures the buffer layer may have multiple materials near the substrate, to reduce defect propagation and control strain, and a single partially or fully strain relaxed material adjacent to the channel layer. The channel layer is generally a continuation of the buffer layer and of the same material as the buffer layer adjacent to the channel layer.
As is also known, Group III-Nitride materials are very strong polar materials with large polarization constants. This strength comes from the strong electronegativity property of the nitrogen atom and the wurtzite crystalline structure these materials naturally form.
As is also know, RF transistors using the Group III-Nitride materials are typically based on AlGaN/GaN heterostructure where at the GaN side of the AlGaN/GaN heterojunction, or interface, a 2DEG forms to compensate the net polarization charge created by the polarization discontinuity that exists at that heterojunction. This 2DEG is the source of the RF transistor current of this structure. In the AlGaN/GaN heterostructure, the AlGaN is the topside barrier separating the heterojunction from the gate electrode while the GaN serves as the channel for which this mobile charge resides. The bandgap of the topside barrier is always larger than the channel layer to mitigate 2DEG spreading into the barrier. In a common source configuration, the potentials at the gate and drain electrodes (the source is typically grounded) of the AlGaN/GaN RF transistor under any operation (on- or off-state, high or low-voltage) cause the mobile sheet charge to spread into the bulk. The degree of the spreading depends highly on the gate and drain voltages applied with the spread being largest under near off state conditions at high drain voltages. This unfortunately is the nominal desired operating conditions for high-power amplifiers. Therefore, this spreading is undesired as it brings deleterious effects such as mobility reduction by scattering and charge loss by trapping via defects and impurities in the bulk of the material. Both of which impact overall gain and power performance under RF operation. In the conventional AlGaN/GaN single heterojunction structures described above, the AlGaN barrier is strained while the GaN channel where the 2DEG resides is not strained.
Prior attempts to confine the charge have utilized a double heterojunction back-barrier design whereby the band structures of the materials are such that the conduction band of the channel layer is lower than both the top barrier layer and the back-barrier layer at the respective heterojunctions. The charge in the channel is then confined by the top and back-barrier layers. Two types of double heterojunction back-barrier HEMTs have generally been used. The first type incorporates a thick back-barrier buffer layer where the crystal structure of the back-barrier layer is partially or fully relaxed and the substrate side of the heterojunction does not contribute to the current in the channel. This structure is shown in FIG. 1 and will be referred to as a single channel double heterojunction back-barrier HEMT. The second type incorporates a thin back barrier layer where the polarization field in the back-barrier creates a second source of electron carriers away from the channel layer. Although this structure, as shown in FIG. 2, confines a 2DEG near the surface, it typically leads to the formation of a second 2DEG that is not confined. This structure is, as noted above, also commonly referred to as a double channel HEMT.
As is also known in the art, exact in-plane lattice matched conditions are difficult to achieve in heterojunction is having epitaxial growth and as a result there usually exists some degree of in-plane mismatch between different layers. When an epitaxial layer is grown on a crystalline substrate or on one or more epitaxial layers with a defined crystallinity, the in-plane lattice of the epitaxial layer will initially conform to match the in-plane lattice constant of the underlying material. However, the epitaxial layer experiences a tensile or compressive in-plane strain as it attempts to conform to the underlying in-plane lattice and the strain energy of the epitaxial layer increases until it becomes large enough to nucleate misfit dislocations. The formation of misfit crystal dislocations reduces the strain in the epitaxial layer and allows the in-plane lattice parameter to relax towards its bulk lattice structure above the interface. The thickness at which misfit dislocations are nucleated to relieve the steam in the epitaxial layer is known as the critical thickness for the layer. The larger the in-plane lattice mismatch, the smaller the critical thickness of the epitaxial layer. When the thickness of the epitaxial layer is less than the critical thickness, the epitaxial layer is said to be pseudomorphic. For Group III-Nitride based transistors, nearly matched in-plane lattices are desired between various layers to minimize misfit dislocations and defect formations.
In the case of single channel double heterojunction back-barrier HEMTs, a thick back-barrier layer is used where the back-barrier layer is partially or fully relaxed and the substrate side of the junction does not contribute significantly to the current in the channel. This case is similar to the single heterojunction HEMT with the exception that the material in the buffer layer adjacent the channel layer is different than the material in the channel layer. In this case, the material in the buffer layer adjacent the channel layer is the back-barrier layer. In this case, there is a lattice mismatch at the channel layer/back-barrier layer heterojunction and the in-plane lattice constant of the channel layer material adjusts to have the lattice parameter of the underlying back-barrier layer to minimize defect formation. The channel layer and the top barrier layer are grown pseudomorphic and that the two layers are strained so that their in-plane lattice parameters match the in-plane lattice parameter of the back-barrier layer.
In the case of the dual channel double heterojunction back-barrier HEMT, the back barrier layer no longer serves the role of the buffer layer and is an additional layer located between the buffer layer and the channel layer. In this case the buffer layer and channel layer are generally the same material and insertion of the back-barrier layer creates additional heterojunctions with both the buffer layer and the channel layer. The in-plane lattice constant of this back-barrier layer will adjust to have the in-plane lattice parameter of the underlying buffer layer material to minimize defect formation. Unlike the case of the single channel back-barrier double heterojunction HEMT, the channel layer in these devices is generally the same material as the buffer layer and is therefore lattice matched to the buffer layer. The barrier layer on the surface side of the channel layer, a so-called top barrier layer (FIG. 2), and the back-barrier layer are strained to maintain the in-plane lattice parameter of the buffer layer. The back-barrier layer and the top barrier layer are grown pseudomorphic in that the two layers are strained so that their in-plane lattice parameters match the in-plane lattice parameter of the buffer layer. In this case, the back-barrier layer confines a 2DEG near the surface, but the additional heterojunction created at the buffer layer/back-barrier layer interface also creates a polarization mismatch leading to the formation of additional charge carriers outside of the confined channel layer. These additional charge carriers can lead to the formation of a second 2DEG. As shown in FIG. 2 these structures contain two channel layers. The first 2DEG is formed by the top barrier/channel layer heterojunction and the electrons are located primarily in first/top channel layer, and confined by the top and back-barrier layers. The second 2DEG is formed by the back-barrier/buffer layer heterojunction and the electrons are located primarily in the buffer layer, which also serves as a second/bottom channel layer, and confined only by the back-barrier layer. Double channel HEMTs can produce large negative threshold voltage shifts, lower transconductance leading to lower gain performance and unwanted parasitic currents. Although these properties may be desirable in some device configurations, they minimize the benefits of the charge confinement.
The carrier concentrations of a semiconductor material may be manipulated during the fabrication process with the addition of impurity atoms, a process commonly referred to as doping. The impurities add allowed energy states in the bandgap of the semiconductor. The impurities which produce allowed states close to the conduction band, typically within a few 100 milli-electron volts (meV), are commonly referred to as donor impurities or n-type dopants. At room temperature the thermal energy is sufficient to excite electrons from the states produced by the donor impurities into the conduction band. Similarly, impurities which produce allowed states close to the valance band, typically within a few 100 meV, are commonly referred to as acceptor impurities or p-type dopants as these allow valance electrons to be thermally excited into the acceptor states producing holes in the valance band. Impurities that produce states in the bandgap that are separated from either the conduction band edge or the valance band edge by more than a few 100 meV are commonly referred to as deep-level impurities. Deep-level impurities can be acceptor-like, donor-like, or may exhibit multiple charge states within the bandgap.